ARM: Kirkwood: fix Makefile.boot

While building the dtbs target, one is getting:

make dtbs
make[1]: *** No rule to make target `arch/arm/boot/kirkwood-qnap-ts219.dtb', needed by `arch/arm/boot/dtbs'.  Stop.
make: *** [dtbs] Error 2

The reason is that there's no kirkwood-qnap-ts219.dts file. Update Makefile.boot
to reflect the dts files present.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>